Read circuit for magnetic tunnel junction (mtj) memory

ABSTRACT

In some embodiments, the present application provides a memory device. The memory device includes a memory cell array comprising a plurality of magnetic tunnel junction (MTJ) memory cells arranged in columns and rows, a read bias circuit connected to the memory cell array and configured to provide a reading bias for a MTJ memory cell of the memory cell array, and a first non-linear resistance device connected in series and between the MTJ memory cell and the read bias circuit. The first non-linear resistance device is configured to provide a first resistance when conducting a first current and a second resistance greater than the first resistance when conducting a second current smaller than the first current.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No. 17/110,624, filed on Dec. 3, 2020, which is a Continuation of U.S. application Ser. No. 16/502,430, filed on Jul. 3, 2019 (now U.S. Pat. No. 10,867,652, issued on Dec. 15, 2020), which claims the benefit of U.S. Provisional Application No. 62/751,994, filed on Oct. 29, 2018. The contents of the above-referenced Patent Applications are hereby incorporated by reference in their entirety.

BACKGROUND

Many modern-day electronic devices contain various kinds of random access memories. A random access memory (RAM) may be a volatile memory where the stored data is lost in the absence of power or a non-volatile memory, which stores data in the absence of power. Resistive or magnetic memory devices including tunnel junctions (MTJs) can be used in RAMs, and are promising candidates for next-generation memory solutions due to simple structures and their compatibility with complementary metal-oxide-semiconductor (CMOS) logic fabrication processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 illustrates a three-dimensional view of some embodiments of MTJ memory cell.

FIG. 2 illustrates a diagram depicting some embodiments of a memory device including an array of MTJ memory cells and associated read circuitry.

FIG. 3 illustrates a block diagram for some embodiments of a reading circuit that can be used in the memory device of FIG. 2.

FIG. 4A illustrates a circuit schematic for some embodiments of a data path that can be used in the memory device of FIG. 2.

FIG. 4B illustrates a circuit schematic for some alternative embodiments of a data path that can be used in the memory device of FIG. 2.

FIGS. 5-6 illustrate timing diagrams depicting some embodiments of reading operations for a memory device, such as the memory device shown in FIGS. 3-4B.

FIG. 7 illustrates a cross-sectional diagram showing some embodiments of a memory device that includes an MTJ memory element.

FIG. 8 illustrates a top view of the memory device of FIG. 7, as indicated by the cut-lines in FIG. 7.

FIG. 9 illustrates a flowchart of some embodiments of a method of reading the MTJ memory device.

FIG. 10 shows an example load line analysis of a series connection of an MTJ memory cell and a forward biased SCR used for reading the MTJ memory device.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples, for implementing different features of this disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein may likewise be interpreted accordingly.

A magnetic tunnel junction (MTJ) includes first and second ferromagnetic films separated by a tunnel barrier layer. One of the ferromagnetic films (often referred to as a “reference layer”) has a fixed magnetization direction, while the other ferromagnetic film (often referred to as a “free layer”) has a variable magnetization direction. For MTJs with positive tunneling magnetoresistance (TMR), if the magnetization directions of the reference layer and free layer are in a parallel orientation, electrons will more likely tunnel through the tunnel barrier layer, such that the MTJ is in a low-resistance state. Conversely, if the magnetization directions of the reference layer and free layer are in an anti-parallel orientation, electrons will less likely tunnel through the tunnel barrier layer, such that the MTJ is in a high-resistance state. Consequently, the MTJ can be switched between two states of electrical resistance, a first state with a low resistance (R_(P): magnetization directions of the reference layer and the free layer are parallel) and a second state with a high resistance (R_(AP): magnetization directions of the reference layer and the free layer are anti-parallel). It is noted that MTJs can also have a negative TMR, e.g., lower resistance for anti-parallel orientation and higher resistance for parallel orientation, and though the following description is written in the context of positive TMR based MTJs, it will be appreciated the present disclosure is also applicable to MTJs with a negative TMR.

Because of their binary nature, MTJs are used in memory cells to store digital data, with the low resistance state R_(P) corresponding to a first data state (e.g., logical “0”), and the high-resistance state R_(AP) corresponding to a second data state (e.g., logical “1”). To read data from such an MTJ memory cell, the MTJ's resistance R_(MTJ) (which can vary between R_(P) and R_(AP), depending on the data state that is stored) can be compared to a reference cell's resistance, R_(Ref) (where R_(Ref), for example, is designed to be in between R_(P) and R_(AP), for instance, an average). In some techniques, a given read voltage V_(Read) is applied to the MTJ memory cell and the reference cell. This read voltage results in a read current flowing through the MTJ (I_(MTJ)) and a reference current flowing through the reference cell (I_(Ref)). If the MTJ is in a parallel state, the read current I_(MTJ) has a first value (I_(MTJ-P)) greater than I_(Ref); while if the MTJ is in an anti-parallel state, the read current I_(MTJ) has a second value (I_(MTJ-AP)) that is less than I_(Ref). Thus, during a read operation, if I_(MTJ) is greater than I_(Ref), then a first digital value (e.g., “0”) is read from the MTJ memory cell. On the other hand, if I_(MTJ) is less than I_(Ref) for the read operation, then a second digital value (e.g., “1”) is read from the MTJ memory cell.

However, MTJ read operation may sometimes also flip or significantly change, and the corresponding probability is called Read Disturb Rate (RDR). RDR, in turn, depends on the magnitude of the current passed through the MTJ (I_(MTJ)) and the duration for which it is passed. Although a large read current would provide good signal separation between R_(P) and R_(AP), a large read current may inadvertently overwrite the free layer in the MTJ. Also, a writing current may also be increased as a result of the large read current. A large writing current would introduce more energy dissipation in the write operation, and may contribute to the chances of MTJ breakdown. Conversely, though a small read current would be less likely to overwrite the free layer, a small read current may provide poor signal separation between R_(P) and R_(AP). As the size of the MTJ is scaled down, the resistance of the MTJ increases and exacerbates these read operation issues. The magnitude of the current passed through the MTJ (I_(MTJ)) depends on an effective TMR of the MTJ memory cell. The effective TMR not only is affected by the resistance of the MTJ but also the resistance of the write path, access transistors, read circuit, etc. In the applications, the effective TMR could be much lower (for instance just one-third) than the actual TMR of the MTJ. In addition, sizes of the MTJ memory cell and reference resistor are scaled down, and resistances of the MTJ memory cell R_(MTJ) and the reference resistor R_(Ref) are scaled up accordingly for successive technology nodes. The difference in the current between the MTJ memory cell and the reference cell, i.e., ΔI (ΔI_(P) or ΔI_(AP) respectively for MTJ in P-state and AP-state) scales down. Hence, as technology nodes advance, the detected signal degrades phenomenally.

In view of above, the present disclosure provides reading circuits and techniques for reading MTJ memory cells that enhance a ratio for the read-current between MTJ's P-state and AP-state beyond a ratio enabled by an effective TMR of an MTJ array TMR_(array), thereby improving the read disturb rate (RDR) while maintaining the pre-designed low write current. One or more non-linear resistors (NLRs) are added to the read system. In some embodiments, a first non-linear resistor (NLR) is connected in series with an MTJ memory cell to enhance the effective TMR by providing a greater resistance when the MTJ memory cell is in a high-resistance (e.g., AP-state) and a smaller resistance when the MTJ memory cell is in a low-resistance state (e.g., P-state). The effective TMR can be designed to be even greater than the TMR of the MTJ itself. In some further embodiments, a second non-linear resistor (NLR) may also be added in series with a reference resistor to improve the readability further. In some embodiments, the non-linear resistor (NLR) can have a current controlled negative resistance, i.e., an S-type negative resistance (NR). An example IV characteristic curve of an S-type negative resistor is shown in FIG. 10. The S-type negative resistor may be a component (e.g., forwardly biased thyristor, SCR, diac, triac, etc.) or an equivalent sub-circuit.

FIG. 1 illustrates some embodiments of a magnetic tunnel junction (MTJ) memory cell 100 that can be used with various read techniques as provided herein. The MTJ memory cell 100 includes a magnetic tunnel junction (MTJ) memory element 102 and an access transistor 104. A bit line (BL) is coupled to one end of the MTJ memory element 102, and a source line (SL) is coupled to an opposite end of the MTJ memory element through the access transistor 104. Thus, application of a suitable word-line (WL) voltage to a gate electrode of the access transistor 104 couples the MTJ memory element 102 between the BL and the SL, and allows a bias to be applied over the MTJ memory element 102 through the BL and the SL. Consequently, by providing suitable bias conditions, the MTJ memory element 102 can be switched between two states of electrical resistance, a first state with a low resistance (the P-state, magnetization directions of the reference layer and the free layer are parallel) and a second state with a high resistance (the AP-state, magnetization directions of the reference layer and free layer are antiparallel), to store data.

In some embodiments, the MTJ memory element 102 comprises the reference layer 106 and a free layer 108 disposed over the reference layer 106 and separated from the reference layer 106 by a barrier layer 110. The reference layer 106 is a ferromagnetic layer that has a magnetization direction that is “fixed”. As an example, the magnetization direction of the reference layer 106 can be “up”, i.e., perpendicular to the plane of the reference layer 106 pointing upwardly along the z-axis. The barrier layer 110, which can manifest as a thin dielectric layer or non-magnetic metal layer in some cases, separates the reference layer 106 from the free layer 108. The barrier layer 110 can be a tunnel barrier which is thin enough to allow quantum mechanical tunneling of current between the reference layer 106 and the free layer 108. In some embodiments, the barrier layer 110 can comprise an amorphous barrier, such as aluminum oxide (AlO_(x)) or titanium oxide (TiO_(x)), or a crystalline barrier, such as manganese oxide (MgO) or a spinel (e.g., MgAl₂O₄). The free layer 108 is capable of changing its magnetization direction between one of two magnetization states, which correspond to binary data states stored in the memory cell. For example, in a first state, the free layer 108 can have an “up” magnetization direction in which the magnetization of the free layer 108 is aligned in parallel with the magnetization direction of the reference layer 106, thereby providing the MTJ memory element 102 with a relatively low resistance. In a second state, the free layer 108 can have a “down” magnetization direction which is aligned and anti-paralleled with the magnetization direction of the reference layer 106, thereby providing the MTJ memory element 102 with a relatively high resistance. The magnetic directions disclosed herein could also be “flipped” or in-plane (e.g., pointing in the x and/or y directions), rather than up-down depending on the implementation. In some embodiments, the free layer 108 may comprise magnetic metal, such as iron, nickel, cobalt, boron, and alloys thereof, for example, such as a CoFeB alloy ferromagnetic free layer. Although this disclosure is described largely in terms of MTJs, it is also to be appreciated that it is applicable to spin valve memory elements, which may use a magnetically soft layer as the free layer 108, and a magnetically hard layer as the reference layer 106, and a non-magnetic barrier separating the magnetically hard layer and magnetically soft layer. The barrier layer 110 of a spin valve is typically a non-magnetic metal. Examples of non-magnetic metals include, but are not limited to: copper, gold, silver, aluminum, lead, tin, titanium and zinc; and/or alloys such as brass and bronze.

A synthetic anti-ferromagnetic (SyAF) layer 105 is disposed under the reference layer 106 or at one side of the reference layer 106 opposite to the free layer 108. The SyAF layer 105 is made of ferromagnetic materials having constrained or “fixed” magnetization directions. This “fixed” magnetization direction can be achieved in some cases by an initializing exposure to a high magnetic field after the entire chip is manufactured. As an example, the SyAF layer 105 may comprise a pair of pinning layers including a first pinning layer 114 and a second pinning layer 118. The first pinning layer 114 and the second pinning layer 118 may have opposite magnetization directions aligned with the magnetization direction of the reference layer 106. Using the same example given above, the first pinning layer has the same “up” magnetization direction with the reference layer. The second pinning layer has an opposite “down” magnetization direction aligned and is anti-paralleled with the magnetization direction of the reference layer 106. An interlayer spacer layer 116 is disposed between the first pinning layer 114 and the second pinning layer 118. The interlayer spacer layer 116 can be an anti-parallel coupling (APC) layer that causes an interexchange coupling (IEC) between the first pinning layer 114 and the second pinning layer 118 such that the first pinning layer 114 and the second pinning layer 118 have anti-parallel magnetic directions and stable each other. As an example, the interlayer spacer layer 116 may comprise ruthenium (Ru) or Iridium (Ir). The first pinning layer 114 may include cobalt layers and nickel layers one stacked above another (Co/Ni)_(m). The first pinning layer 114 may also be cobalt palladium stack (Co/Pd)_(m), or cobalt platinum stack (Co/Pt)_(m), where m can be a positive integer. The second pinning layer 118 may comprise a reverse of the compositions of the first pinning layer 114 with the same or different amount of layers. For example, the second pinning layer 118 may include nickel layers and cobalt layers one stacked above another (Ni/Co)_(n), or palladium cobalt stack ((Pd/Co)_(n), or platinum cobalt stack (Pt/Co)_(n), where n can be a positive integer. A transition layer 112 may be disposed between the first pinning layer 114 and the reference layer 106. The transition layer 112 is made of non-magnetic materials and is configured as a buffer layer, a lattice match layer, and/or a diffusion barrier. As an example, the transition layer 112 may comprise tantalum (Ta), tungsten (W), molybdenum (Mo), Hafnium (Hf), or CoFeW.

FIG. 2 illustrates a memory device 200 that includes a number of MTJ memory cells 100 according to some embodiments of the present disclosure. Each MTJ memory cell 100 includes an MTJ memory element 102 and an access transistor 104. The MTJ memory cells 100 are arranged in M columns (bits), and N rows (words), and are labeled CROW-COLUMN in FIG. 2. Word-lines (WL) extend along respective rows and are coupled to gate electrodes of the access transistors 104 along the respective rows. Bit lines (BL) and source lines (SL) extend along respective columns, with the BLs being coupled to the free layers of the MTJ memory elements 102, and the SLs being coupled to the reference layers of the MTJ memory elements 102 through the access transistors 104. For example, in Row 1 of the memory device 200, the cells C₁₋₁ through C_(M-1) form an M-bit data word accessible by activation of word-line WL1. Thus, when WL1 is activated, data states can be written to or read from the respective cells C₁₋₁ through C_(M-1) through bit lines BL₁ through BL_(M) and/or by source lines SL₁ through SL_(M). Each column also has a sense amplifier (S/A) that is used to detect a stored data state from an accessed cell of the column during a read operation. Thus, the data in the accessed cells is sensed using sense amp circuits 202 (S/A C₁ through S/A C_(M)) associated with columns 1 through M, respectively. For example, when WL₁ is activated (other WLs are deactivated), the bit lines (BL₁ through BL_(M), respectively) develop respective biases corresponding to the respective data states stored in the accessed memory cells (C₁₋₁ through C_(M-1), respectively); and the sense amps (S/A C₁ through S/A C_(M), respectively) detect the data states from the bit lines (BL₁ through BL_(M), respectively).

During a typical write operation to Row 1, a voltage V_(WL) is applied to a word-line WL₁, wherein the V_(WL) is typically greater than or equal to a threshold voltage of the access transistors 104, thereby turning on the access transistors within Row 1 and coupling the bit lines BL₁-BL_(M) to the MTJ memory elements 102 in the accessed cells (e.g., memory cells C₁₋₁ through C_(1-M)). Suitable voltages are applied to the bit lines BL₁-BL_(M) and source lines SL₁-SL_(M), where the voltage on each bit line is representative of a data value to be written to the memory cell attached to that bit line. While Row1 is accessed, the word-lines of the other rows (WL₂-WL_(N)) remain off, such that the MTJ memory elements of the other cells remain isolated and are not written to or read from.

During a typical read operation of Row 1, voltage V_(WL) is again applied to word-line WL₁ to turn on the access transistors 104 and couple the bit lines BL₁ through BL_(M) to the MTJ memory elements of the accessed cells (C₁₋₁ through C_(1-M)). The MTJ memory elements then discharge charge through the access transistors 104 to the bit lines BL₁ through BL_(M), based on their stored states, thereby causing the bit line voltages BL₁-BL_(M) to change. The amount by which the bit line voltages change depends upon the state of the MTJ memory elements 102 being accessed. To determine whether the state of the MTJ memory elements being accessed is a “1” or a “0”, one differential input terminal of each sense amp 202 is coupled to the bit line of the column (e.g., S/A C1 is coupled to bit line BL₁) and the other differential sense amp input is coupled to a reference voltage (e.g., reference bit line REFBL₁ in this example). Depending upon whether the cell bit line BL₁ is high or low relative to the reference voltage on REFBL₁, the sense amp returns a “1” or a “0”.

It will be appreciated that current can flow in various directions depending on the implementation. In some embodiments, read current flows from the BL to the SL. However, a backward read can also occur in other embodiments, in which read current flows from the SL to the BL. Also, the entire MTJ structure can be fabricated upside down and is called top-pinning MTJ. Hence, in the case of a top-pinning MTJ, the BL is nearer the reference layer 106, and the SL is nearer the free layer 108.

FIG. 3 illustrates a block diagram for some embodiments of a reading circuit 300 that can be used in the memory device 200 of FIG. 2. For simplicity, an MTJ memory cell 100 is shown in FIG. 3, though it will be appreciated that additional memory cells can be arranged in parallel with the illustrated MTJ memory cell 100 via a bit line BL and a source line SL consistent with FIG. 2. The reading circuit 300 comprises a read bias circuit 302. During a read operation, the read bias circuit 302 provides a reading voltage V_(read) for the MTJ memory cell 100 and a reference cell 100′ and accordingly output an output signal. A current mirror circuit may be used as a load of the read bias circuit. A sense amplifier 304 may be used to generate a digital output signal by processing output signals of the read bias circuit 302. For example, the read bias circuit 302 may sense a read current I_(MT)J flowing through the MTJ memory cell 100 and a reference current I_(Ref) flowing through the reference cell and generate a sensing voltage V_(_mtj) and a reference voltage V_(_ref) to feed into the sense amplifier 304. A read enable circuit 308 can pull up a voltage level (e.g., a voltage level on the bit line BL) during the read operation, and a pull-down circuit 308 can pull down a voltage level (e.g., a voltage level on the source line SL) during the read operation.

A first non-linear resistor (NLR) device 310 is coupled to the MTJ memory cell 100 in series and provides a transmission path for the read current I_(MTJ). The first NLR device 310 may be connected between the read bias circuit 302, and the read enable circuit 306. The first NLR device 310 is configured to provide a resistance that provides adjustment for a current flowing through the MTJ memory cell 100. The resistance of the first NLR device 310 may decrease as the voltage applied on the first NLR device 310 increases. In some embodiments, the first NLR device 310 is an S-type negative resistance (NR) such as a forwardly biased thyristor (e.g., silicon control rectifier (SCR), diac, triac, etc.).

In some further embodiments, a second NLR device 312 is also coupled to the reference cell 100′ in series and provides adjustment for a reference current I_(ref). The second NLR device 312 may be connected between the read bias circuit 302, and the read enable circuit 306 in parallel with the first NLR device 310. The second NLR device 312 provides an adjustment to the reference current I_(REF) such that the reference current I_(REF) fall within the range between the read current of P-state and AP-state. The second NLR device 312 may have same or similar features as the first NLR device 310. As an example, for S-type negative resistance (NR) such as a forwardly biased thyristor (e.g., SCR, diac, triac, etc.), a reverse-biased zener diode, or equivalent transistor circuits, an additional NLR may not be needed for the reference cell since the separation between the R_(AP)+R_(NLR) and R_(P)+r_(NLR) should be large. On the other hand, the second NLR device can be more beneficial for a forward-biased conventional diode (e.g., pn-diode, Schottky diode) or equivalent transistor circuit, since R_(NLR) and r_(NLR) could be quite close, and therefore NLR should be added in the read path of reference cell as well.

FIG. 4A and FIG. 4B illustrate schematic views of data paths 400 a and 400 b of the memory array in more detail. The data path 400 a or 400 b corresponds to a single column of the memory array of FIG. 2, albeit along with some standard additional circuitry which was omitted from FIG. 2 for simplicity. For clarity, the data path 400 a or 400 b is illustrated with only a single MTJ memory cell 100, though it will be appreciated that additional memory cells can be arranged in parallel with the illustrated MTJ memory cell 100 via BL and SL consistent with FIG. 2.

The data path 400 a includes an MTJ current path 402 and a reference current path 404, which are arranged in parallel with one another between V_(DD) and V_(SS). A read bias circuit 302 can be a differential amplifier. The read bias circuit 302 may include a current mirror circuit including transistors M3, M2 used as a load for the MTJ current path 402 and the reference current path 404. Transistors M4 and M6 can be driven by the same input voltage V4 from an equalizer. A read enable circuit 306 may include transistors M5, M7 that respectively pulls up a voltage level for the MTJ current path 402 and the reference current path 404 during the read operation. A pull-down circuit 308 may include transistors M8, M10 that respectively pulls down a voltage level for the MTJ current path 402 and the reference current path 404 during the read operation. The read enable circuit 306 and the pull-down circuit 308 cut off the read circuit when the read operation is not required. A sense amplifier 304 may include a differential amplifier having transistors M11-M15. M13 and M 14 are driven by different voltages V_(_mtj) and V_(_ref). M12 and M11 serve as current mirror load. The voltage outputs of M13 and M14 are sensed at the respective drain terminals. V01 is fed into, for instance, an inverter which acts as a simple sense amplifier, shapes waveform and ensures correct polarity of the output in this implementation. The sense amplifier 304 is configured to detect a data state from the MTJ memory cell 100 by comparing a voltage provided by the MTJ memory cell 100 (V_(_mtj)) with a reference voltage (V_(_ref)) provided by a reference cell 100′. Based on these voltages (V_(_mtj), V_(_Ref)), the sense amplifier 304 provides an output voltage (V_(_Out)) that is in one of two states, representing a logical “1” or a logical “0”, which was stored in the accessed memory cell 100.

The MTJ current path 402 includes a first current mirror transistor M3, a first pull-up read-enable transistor M7, the MTJ memory cell 100 (including an MTJ memory element MTJ and a first access transistor M1), and a first pull-down read-enable transistor M8. Bit line (BL) and source line (SL) are coupled to opposite ends of the MTJ memory cell 100. The BL is coupled to the MTJ memory element MTJ, and the SL is coupled to the first access transistor M1 and is separated from the MTJ memory element MTJ by the first access transistor M1. The reference current path 404 includes a second current mirror transistor M2; a second pull-up read-enable transistor M5; the reference cell 100′ (including a reference MTJ memory element Ref, which can be implemented as a resistor with a fixed resistance in some embodiments, and a second access transistor M9); and a second pull-down read-enable transistor M10. A reference bit line (BL_(Ref)) and reference source line (SL_(Ref)), which have lengths and resistances that are substantially equal to those of the BL and SL, are coupled to opposite ends of the reference cell 100′. The BL_(Ref) is coupled to the reference MTJ memory element Ref, and the SL_(Ref) is coupled to the second access transistor M9 and is separated from the reference MTJ memory element Ref by the second access transistor M9.

Control signals are provided to a word-line node WL and a read-enable node RE to facilitate read and write operations. The word-line node WL may be biased by a voltage source V2, and the read-enable node RE may be biased by a voltage source V3 during read and write operations. The word-line node WL is coupled to respective gates of the first access transistor M1 and the second access transistor M9. The read-enable node RE is coupled to respective gates of the pull-up transistors M7, M5, and the pull-down transistors M8, M10. The read-enable node RE is typically low (e.g., 0 volts) during write operations, and is typically high (V_(DD)) during read operations.

A first NLR device 310 is coupled in the MTJ current path 402. The first NLR device 310 may be connected in series and between the first pull-up read-enable transistor M7 and the first current mirror transistor M3. The first NLR device 310 is configured to provide a resistance that provides adjustment for a current flowing through the MTJ current path 402. The resistance of the first NLR device 310 may decrease as the voltage applied on the first NLR device 310 increases, and thus increase an effective tunnel magnetoresistance (TMR) of the MTJ memory cell. TMR of an MTJ memory cell is defined as (R_(AP)−R_(P))/(R_(Path)+R_(P)+R_(MOS))=(I_(P)−I_(AP))/I_(AP), where R_(AP) is the electrical resistance of the MTJ element in the anti-parallel state; R_(P) is the resistance of the MTJ element in the parallel state; R_(Path) is the resistance of the write path; R_(MOS) is the resistance of the access transistor; I_(P) is the current in the parallel state; and I_(AP) is the current in the anti-parallel state.

Using an MTJ with positive tunneling magnetoresistance (TMR) as an example for illustration. If the magnetization directions of the reference layer and free layer are in a parallel orientation, the MTJ is in a low-resistance state (P-state). If the magnetization directions of the reference layer and free layer are in an anti-parallel orientation, the MTJ is in a high-resistance state (AP-state). The insertion of the first NLR device 310 increases I_(p) and decrease I_(ap), and thus increase TMR. The first NLR device 310 provides a first resistance (r_(nlr)) when the low-resistance state P-state is read and a second resistance (R_(nlr)) greater than the first resistance (r_(nlr)) when the high-resistance state AP-state is read. Thus, the difference between I_(p) and I_(ap) is increased. The effective TMR becomes: {(R_(AP)−R_(P))+(R_(nlr)−r_(nlr))}/(R_(Path)+R_(P)+R_(MOS)+r_(SD)). The insertion of the first NLR device 310 also provides more margin to design the reference cell 100′. The reference resistor R_(ref) would be in a range between R_(AP)+R_(nlr) and R_(P)+r_(nlr), instead of in a smaller range between R_(AP) and R_(P). The reference cell 100′ has a reference resistance greater than a sum of the first resistance (R_(P)) of the MTJ memory cell 100 and the first resistance (r_(nlr)) of the first NLR device 310 and smaller than a sum of the second resistance (RAP) of the MTJ memory cell 100 and the second resistance (R_(nlr)) of the first NLR device 310.

In addition, the insertion of the first NLR device 310 reduces the RDR for forward read direction, as the read current for AP-state is reduced. To maintain the same charging for P-state, the read voltage needs to be increased. There could be at least following three ways: increase the V_(Read); increase a gate voltage VG of the access transistor; or increase both V_(Read) and V_(G).

Similarly, in some further embodiments, a second NLR device 312 is also coupled to the reference cell 100′ in series and provides adjustment for a reference current I_(ref). The second NLR device 312 may be connected between the read bias circuit 302 and the read enable circuit 306 in parallel with the first NLR device 310. The second NLR device 312 may have same or similar features as the first NLR device 310.

FIG. 4B shows the data path 400 b. Compared to the data path 400 a in FIG. 4A, the second NLR device 312 is not present. Thus M4 is connected to M5 while M6 is separated from M7 by the first NLR device 310. As an example, for S-type negative resistance (NR) such as a forward biased thyristor (e.g. SCR, diac, triac, etc), a reverse-biased zener diode, or equivalent transistor circuits, there shouldn't be a need of NLR for the reference cell 100′ since the separation between the R_(AP)+R_(NLR) and R_(P)+r_(NLR) should be large; but for a forward-biased conventional diode (e.g., pn-diode, Schottky diode) or equivalent transistor circuit, since R_(NLR) and r_(NLR) could be quite close, NLR should be added in read path of reference cell as well.

Referring now to FIG. 5, a description of some embodiments of how the data paths 400 a, 400 b can operate during read operations is provided with regards to a timing/waveform diagram. FIG. 5 shows waveforms for two read operations on a single MTJ memory cell superimposed over one another to show how the current and voltage levels relate to one another. FIG. 6 shows waveforms for two read operations of a reading operation without an NLR device for comparison purpose. For a first read operation, the MTJ is in a parallel state, such that the first read operation returns a low voltage (e.g., logical “0”). For the second read operation, the MTJ is in an anti-parallel state, such that the second read operation returns a high voltage (e.g., logical “1”). As shown in FIG. 5 and FIG. 6, when V(re) is active to enable read operation, V(scr_gate) is active, and V_mtj changes in response to I(Mtj). SA may generate V_out according to V01, which is changed in response to V_mtj. For a comparison circuit without NLR devices shown in FIG. 6, I_(P) is 50.6 μA; I_(AP) is 44.1 μA, and thus a sensed TMR is around 14.74%. A read time is about 7.4 ns. The SCR gate voltage is tuned to make sure that P-state current IP is same for comparison purpose. From simulated waveforms shown in FIG. 6, I_(P) in the disclosed reading operation is 50.4 μA; I_(AP) is 32.9 μA, and thus, the sensed TMR is around 53.19%. Also seen from the waveforms, a read time is about 5.4 ns. Thus, AP-state current I_(AP) of the disclosed reading operation of FIG. 5 is reduced, and TMR for the proposed circuit is relatively high as compared to that of the reading operation of FIG. 6. Also, the disclosed circuit can perform read operations at higher read speed.

FIG. 7 illustrates a cross-sectional view of some embodiments of an integrated circuit 700, which includes MTJ memory elements 102 a, 102 b disposed in an interconnect structure 704 of the integrated circuit 700. The integrated circuit 700 includes a substrate 706. The substrate 706 may be, for example, a bulk substrate (e.g., a bulk silicon substrate) or a silicon-on-insulator (SOI) substrate. The illustrated embodiment depicts one or more shallow trench isolation (STI) regions 708, which may include a dielectric-filled trench within the substrate 706.

Two access transistors 710, 712 are disposed between the STI regions 708. The access transistors 710, 104 include access gate electrodes 714, 716, respectively; access gate dielectrics 718, 720, respectively; access sidewall spacers 722; and source/drain regions 724. The source/drain regions 724 are disposed within the substrate 706 between the access gate electrodes 714, 716 and the STI regions 708, and are doped to have a first conductivity type which is opposite a second conductivity type of a channel region under the gate dielectrics 718, 720, respectively. The word line gate electrodes 714, 716 may be, for example, doped polysilicon or a metal, such as aluminum, copper, or combinations thereof. The word line gate dielectrics 718, 720 may be, for example, an oxide, such as silicon dioxide, or a high-□ dielectric material. The word line sidewall spacers 722 can be made of silicon nitride (e.g., Si₃N₄), for example.

The interconnect structure 704 is arranged over the substrate 706 and couples devices (e.g., access transistors 710, 104) to one another. The interconnect structure 704 includes a plurality of IMD layers 726, 728, 730, and a plurality of metallization layers 732, 734, 736 which are layered over one another in alternating fashion. The IMD layers 726, 728, 730 may be made, for example, of a low κ dielectric, such as un-doped silicate glass, or an oxide, such as silicon dioxide. The metallization layers 732, 734, 736 include metal lines 738, 740, 742, which are formed within trenches, and which may be made of a metal, such as copper or aluminum. Contacts 744 extend from the bottom metallization layer 732 to the source/drain regions 724 and/or gate electrodes 714, 716; and vias 746 extend between the metallization layers 732, 734, 736. The contacts 744 and the vias 746 extend through dielectric-protection layers 750, 752 (which can be made of dielectric material and can act as etch stop layers during manufacturing). The dielectric-protection layers 750, 752 may be made of an extreme low-□ dielectric material, such as SiC, for example. The contacts 744 and the vias 746 may be made of a metal, such as copper or tungsten, for example.

MTJ memory elements 102 a, 102 b, which are configured to store respective data states, are arranged within the interconnect structure 704 between neighboring metal layers. The MTJ memory element 102 a includes an MTJ, including a pinning layer 114, a metallic interlayer 116, a reference layer 106, a barrier layer 110, and a free layer 108.

FIG. 8 depicts some embodiments of a top view of FIG. 7's integrated circuit 700 as indicated in the cut-away lines shown in FIGS. 7-8. As can be seen, the MTJ memory elements 102 a, 102 b can have a square/rectangular or circular/elliptical shape when viewed from above in some embodiments. In other embodiments, however, for example, due to practicalities of many etch processes, the corners of the illustrated square shape can become rounded, resulting in MTJ memory elements 102 a, 102 b having a square shape with rounded corners, or having a circular shape. The MTJ memory elements 102 a, 102 b are arranged over metal lines 740, respectively, and have upper portions in direct electrical connection with the metal lines 742, respectively, without vias or contacts there between in some embodiments. In other embodiments, vias or contacts couple the upper portion to the metal lines 742.

FIG. 9 illustrates a flowchart 900 of some embodiments of a method of reading from an MTJ memory cell.

At act 902, a memory device is provided. The memory device includes a magnetic tunnel junction (MTJ) current path and a reference current path in parallel with the MTJ current path. The MTJ current path comprises an MTJ memory cell connected in series with a non-linear resistance device. In some embodiments, this memory device can, for example, correspond to the memory device and the data path illustrated in FIGS. 1-4B.

At act 904, a reading voltage (V_(READ)) is provided to generate an MTJ current (I_(MTJ)) through the MTJ current path and to generate a reference current (I_(REF)) through the reference current path. In some embodiments, the MTJ current can correspond, for example, to signal I_(MTJ) in FIG. 5, and the reference current can correspond, for example, to signal I_(Ref) in FIG. 5.

At act 906, the reference current I_(REF) and the MTJ current I_(MTJ) are compared with one another to determine a status of the MTJ memory cell between a first data state having a first resistance and a second data state having a second resistance. The first data state differs from the second data state.

At act 908, a differential current between the memory current path and the reference current path is sensed. A voltage detection signal is detected based on the sensed differential current.

At act 910, the voltage detection signal is buffered to output a digital signal indicating a data state of the MTJ memory device.

FIG. 10 shows an example load line analysis of a series connection of an MTJ memory cell 100 and a forward biased SCR device as the first NLR device 310. The quiescent points for the P-state and the AP-state of the MTJ memory cell are shown in the figure as V₁/R_(P) and V₁/R_(AP). The IV curve of a negative resistance device including the SCR device has a region where for the differential increase in voltage is proportional to a differential decrease in current through the device, and vice versa, i.e., the IV characteristics have a negative slope. Note that this negative slope region slope is unstable. Therefore, the device operating points reside in the region of the positive slopes on either sides of the negative slope region. The operating points for the P-state and the AP-state are chosen to be different regions on either side of the negative region. Other NLR devices used for the disclosed reading path may operate similarly. The SCR device offers small resistance r_(SCR) for P-state of the MTJ, while the SCR device offers a large resistance R_(SCR) for AP-state. Therefore, the net resistance between the read voltage and the ground for P-state and AP-state respectively becomes: R_(Path)+R_(P)+R_(MOS)+r_(SCR) and R_(Path)+R_(AP)+R_(MOS)+R_(SCR). Therefore, the effective TMR is (R_(AP)−R_(P))/(R_(Path)+R_(P)+R_(MOS)) without SCR. While the new effective TMR is {(R_(AP)−R_(P))+(R_(SCR)−r_(SCR))}/(R_(Path)+R_(P)+R_(MOS)+r_(SCR)) after adding SCR. Thereby, the effective TMR can be improved, and thus makes it much easier to detect the difference in the read currents for P-state and AP-state.

Thus, in some embodiments, the present application provides a memory device. The memory device includes a memory cell array comprising a plurality of magnetic tunnel junction (MTJ) memory cells arranged in columns and rows, a read bias circuit connected to the memory cell array and configured to provide a reading bias for a MTJ memory cell of the memory cell array, and a first non-linear resistance device connected in series and between the MTJ memory cell and the read bias circuit. The first non-linear resistance device is configured to provide a first resistance when conducting a first current and a second resistance greater than the first resistance when conducting a second current smaller than the first current.

In other embodiments, the present application provides a memory device. The memory device includes an MTJ memory cell configured to switch between a first data state and a second data state, a reference cell coupled in parallel with the MTJ memory cell, and a read bias circuit connected to the memory cell and the reference cell and configured to provide a reading bias respectively for the MTJ memory cell and the reference cell. The first data state has a first resistance and the second data state has a second resistance greater than the first resistance. The memory device further includes a first non-linear resistance device connected in series and between the MTJ memory cell and the read bias circuit. The first non-linear resistance device is configured to provide a first resistance when conducting a first current and a second resistance greater than the first resistance when conducting a second current smaller than the first current.

In yet other embodiments, the present disclosure provides a method for reading from a memory device. In the method, a magnetic tunnel junction (MTJ) current path is provided, and a reference current path is provided in parallel with the MTJ current path. The MTJ current path comprises an MTJ memory cell connected in series with a non-linear resistance device. A reading bias is provided to generate an MTJ current through the MTJ current path and to generate a reference current through the reference current path. The reference current and the MTJ current are compared to generated a digital signal indicating a data state of the MTJ memory cell.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

1. A memory device, comprising: a memory cell array comprising a plurality of magnetic tunnel junction (MTJ) memory cells arranged in columns and rows; a read bias circuit connected to the memory cell array and configured to provide a reading bias for a MTJ memory cell of the memory cell array; and a first non-linear resistance device connected in series and between the MTJ memory cell and the read bias circuit, wherein the first non-linear resistance device is configured to provide a first resistance when conducting a first current and a second resistance greater than the first resistance when conducting a second current smaller than the first current.
 2. The memory device of claim 1, wherein the first non-linear resistance device is an S-type negative resistor or an equivalent sub-circuit.
 3. The memory device of claim 1, wherein the first non-linear resistance device is a silicon control rectifier (SCR) or a triac.
 4. The memory device of claim 1, further comprising: a reference cell connected in parallel with the MTJ memory cell; and a second non-linear resistance device connected in series and between the reference cell and the read bias circuit.
 5. The memory device of claim 4, wherein the read bias circuit comprises: a first read bias transistor connected in series with the MTJ memory cell; and a second read bias transistor connected in series with the MTJ memory cell.
 6. The memory device of claim 5, further comprising: a first read-enable transistor connected in series with the first read bias transistor; and a second read-enable transistor in series with the second read bias transistor.
 7. The memory device of claim 5, further comprising: a sense amplifier having a first input terminal and a second input terminal that are configured to receive a differential input signal, the first input terminal coupled to a first node of the first read bias transistor and the second input terminal coupled to a first node of the second read bias transistor.
 8. The memory device of claim 7, wherein the first node of the first read bias transistor is connected to a first current mirror transistor; and wherein the first node of the second read bias transistor is connected to a second current mirror transistor.
 9. The memory device of claim 1, wherein the MTJ memory cell is configured to switch between a first data state and a second data state, the first data state having a first resistance and the second data state having a second resistance greater than the first resistance.
 10. The memory device of claim 1, further comprising a source line and a bit line respectively coupled to opposite terminals of the MTJ memory cell.
 11. The memory device of claim 10, wherein the MTJ memory cell comprises a MTJ memory element and an access device connected in series, and wherein the MTJ memory element comprises: a ferromagnetic free layer connected to the bit line; a ferromagnetic reference layer connected to the source line; and a non-magnetic barrier layer disposed between and separating the ferromagnetic reference layer and the ferromagnetic free layer.
 12. A memory device, comprising: an MTJ memory cell configured to switch between a first data state and a second data state, the first data state having a first resistance and the second data state having a second resistance greater than the first resistance; a reference cell coupled in parallel with the MTJ memory cell; a read bias circuit connected to the MTJ memory cell and the reference cell and configured to provide a reading bias respectively for the MTJ memory cell and the reference cell; and a first non-linear resistance device connected in series and between the MTJ memory cell and the read bias circuit, wherein the first non-linear resistance device is configured to provide a first resistance when conducting a first current and a second resistance greater than the first resistance when conducting a second current smaller than the first current.
 13. The memory device of claim 12, wherein the first non-linear resistance device comprises an S-type negative resistor.
 14. The memory device of claim 12, further comprising a second non-linear resistance device connected in series and between the reference cell and the read bias circuit.
 15. The memory device of claim 14, wherein the reference cell having a reference resistance greater than a sum of the first resistance of the MTJ memory cell and the first resistance of the first non-linear resistance device; and wherein the reference resistance of the reference cell is smaller than a sum of the second resistance of the MTJ memory cell and the second resistance of the first non-linear resistance device.
 16. The memory device of claim 12, further comprising a read-enable transistor coupled between the first non-linear resistance device and the MTJ memory cell and configured to enable or cut off a reading operation.
 17. The memory device of claim 12, further comprising a sense amplifier having a first input terminal coupled to the MTJ memory cell and a second input terminal coupled to the reference cell and configured to receive a differential input signal and generate an output voltage indicating one of the first data state and the second data state of the MTJ memory cell.
 18. A method for reading from an MTJ memory device, comprising: providing a magnetic tunnel junction (MTJ) current path and a reference current path in parallel with the MTJ current path, wherein the MTJ current path comprises an MTJ memory cell connected in series with a non-linear resistance device; providing a reading bias to generate an MTJ current through the MTJ current path and to generate a reference current through the reference current path; and comparing the reference current and the MTJ current to generated a digital signal indicating a data state of the MTJ memory cell.
 19. The method of claim 18, wherein the non-linear resistance device provides a first resistance when the MTJ memory device is in a low-resistance state and a second resistance greater than the first resistance when the MTJ memory device is in a high-resistance state.
 20. The method of claim 18, wherein operating points of the non-linear resistance device for a first data state and a second data state of the MTJ memory cell are chosen to be on opposite sides of a negative region of an IV characteristic curve of the non-linear resistance device. 